How to customize getattr(obj, prop) function ?

[Pierre]

Hi,

Sorry in advance, english is not my main language :/

I'd like to customize the result obtained by getattr on an object : if
the object has the requested property then return it BUT if the object
doesn't has actually this property return something else.

In my case, I can't use getattr(object, property, default_value).

I tried to write a class with a __getattr__ method and even a
__getattribute_ _ method but this doesn't do what I want....

Maybe I didn't correctly understand this :

http://docs.python.org/ref/attribute-access.html

Here is a piece of my code :
=============== =============== =======
class myclass:
"""docstrin g"""

a = 'aa'
b = 'bb'

def __getattr___(se lf, ppt):
"""getattr" ""
if hasattr(self, ppt):
return self.ppt
else:
return "my custom computed result"

def __getattribute_ _(self, ppt):
"""getattribute """
if hasattr(self, ppt):
return self.ppt
else:
return "my custom computed result"

if __name__ == "__main__":

d = myclass()
p1 = getattr(d, "a")
print p1
p2 = getattr(d, "b")
print p2
p3 = getattr(d, "c")
print p3
=============== =============== ==

I get an AttributeError when accessing to the property named "c".

Any explanation/solution to my problem ?

[Richard Copeland]

Re: How to customize getattr(obj, prop) function ?


Try this:

class Class:
a='aa'
b='bb'
def __getattr__(sel f, ppt):
return 'custom computed result'

__getattr__ is only called when "normal" attribute lookup fails, so
there's no need for the "hasattr" test.

Also, I believe that __getattribute_ _ only applies to new-style classes
(classes derived from "object"), and it will override *all* attribute
accesses, anyway (meaning you'd have infinite recursion if you used it
since you look up "self.ppt")

Pierre wrote:[color=blue]
> Hi,
>
> Sorry in advance, english is not my main language :/
>
> I'd like to customize the result obtained by getattr on an object : if
> the object has the requested property then return it BUT if the object
> doesn't has actually this property return something else.
>
> In my case, I can't use getattr(object, property, default_value).
>
> I tried to write a class with a __getattr__ method and even a
> __getattribute_ _ method but this doesn't do what I want....
>
> Maybe I didn't correctly understand this :
> http://docs.python.org/ref/attribute-access.html
>
> Here is a piece of my code :
> =============== =============== =======
> class myclass:
> """docstrin g"""
>
> a = 'aa'
> b = 'bb'
>
> def __getattr___(se lf, ppt):
> """getattr" ""
> if hasattr(self, ppt):
> return self.ppt
> else:
> return "my custom computed result"
>
> def __getattribute_ _(self, ppt):
> """getattribute """
> if hasattr(self, ppt):
> return self.ppt
> else:
> return "my custom computed result"
>
> if __name__ == "__main__":
>
> d = myclass()
> p1 = getattr(d, "a")
> print p1
> p2 = getattr(d, "b")
> print p2
> p3 = getattr(d, "c")
> print p3
> =============== =============== ==
>
> I get an AttributeError when accessing to the property named "c".
>
> Any explanation/solution to my problem ?
>[/color]

[Richard Copeland]

Re: How to customize getattr(obj, prop) function ?

Try this:

class Class:
a='aa'
b='bb'
def __getattr__(sel f, ppt):
return 'custom computed result'

__getattr__ is only called when "normal" attribute lookup fails, so
there's no need for the "hasattr" test.

Also, I believe that __getattribute_ _ only applies to new-style classes
(classes derived from "object"), and it will override *all* attribute
accesses, anyway (meaning you'd have infinite recursion if you used it
since you look up "self.ppt")

Pierre wrote:[color=blue]
> Hi,
>
> Sorry in advance, english is not my main language :/
>
> I'd like to customize the result obtained by getattr on an object : if
> the object has the requested property then return it BUT if the object
> doesn't has actually this property return something else.
>
> In my case, I can't use getattr(object, property, default_value).
>
> I tried to write a class with a __getattr__ method and even a
> __getattribute_ _ method but this doesn't do what I want....
>
> Maybe I didn't correctly understand this :
> http://docs.python.org/ref/attribute-access.html
>
> Here is a piece of my code :
> =============== =============== =======
> class myclass:
> """docstrin g"""
>
> a = 'aa'
> b = 'bb'
>
> def __getattr___(se lf, ppt):
> """getattr" ""
> if hasattr(self, ppt):
> return self.ppt
> else:
> return "my custom computed result"
>
> def __getattribute_ _(self, ppt):
> """getattribute """
> if hasattr(self, ppt):
> return self.ppt
> else:
> return "my custom computed result"
>
> if __name__ == "__main__":
>
> d = myclass()
> p1 = getattr(d, "a")
> print p1
> p2 = getattr(d, "b")
> print p2
> p3 = getattr(d, "c")
> print p3
> =============== =============== ==
>
> I get an AttributeError when accessing to the property named "c".
>
> Any explanation/solution to my problem ?
>[/color]

[Richard Copeland]

Re: How to customize getattr(obj, prop) function ?

Try this:

class Class:
a='aa'
b='bb'
def __getattr__(sel f, ppt):
return 'custom computed result'

__getattr__ is only called when "normal" attribute lookup fails, so
there's no need for the "hasattr" test.

Also, I believe that __getattribute_ _ only applies to new-style classes
(classes derived from "object"), and it will override *all* attribute
accesses, anyway (meaning you'd have infinite recursion if you used it
since you look up "self.ppt")

Pierre wrote:[color=blue]
> Hi,
>
> Sorry in advance, english is not my main language :/
>
> I'd like to customize the result obtained by getattr on an object : if
> the object has the requested property then return it BUT if the object
> doesn't has actually this property return something else.
>
> In my case, I can't use getattr(object, property, default_value).
>
> I tried to write a class with a __getattr__ method and even a
> __getattribute_ _ method but this doesn't do what I want....
>
> Maybe I didn't correctly understand this :
> http://docs.python.org/ref/attribute-access.html
>
> Here is a piece of my code :
> =============== =============== =======
> class myclass:
> """docstrin g"""
>
> a = 'aa'
> b = 'bb'
>
> def __getattr___(se lf, ppt):
> """getattr" ""
> if hasattr(self, ppt):
> return self.ppt
> else:
> return "my custom computed result"
>
> def __getattribute_ _(self, ppt):
> """getattribute """
> if hasattr(self, ppt):
> return self.ppt
> else:
> return "my custom computed result"
>
> if __name__ == "__main__":
>
> d = myclass()
> p1 = getattr(d, "a")
> print p1
> p2 = getattr(d, "b")
> print p2
> p3 = getattr(d, "c")
> print p3
> =============== =============== ==
>
> I get an AttributeError when accessing to the property named "c".
>
> Any explanation/solution to my problem ?
>[/color]

[Pierre]

Re: How to customize getattr(obj, prop) function ?

Nop I get the same AttributeError when I try to access to the property
"c"...

[George Sakkis]

Re: How to customize getattr(obj, prop) function ?

Pierre wrote:
[color=blue]
> Hi,
>
> Sorry in advance, english is not my main language :/
>
> I'd like to customize the result obtained by getattr on an object : if
> the object has the requested property then return it BUT if the object
> doesn't has actually this property return something else.
>
> In my case, I can't use getattr(object, property, default_value).
>
> I tried to write a class with a __getattr__ method and even a
> __getattribute_ _ method but this doesn't do what I want....
>
> Maybe I didn't correctly understand this :
> http://docs.python.org/ref/attribute-access.html
>
> Here is a piece of my code :
> =============== =============== =======
> class myclass:
> """docstrin g"""
>
> a = 'aa'
> b = 'bb'
>
> def __getattr___(se lf, ppt):
> """getattr" ""
> if hasattr(self, ppt):
> return self.ppt
> else:
> return "my custom computed result"[/color]

1) You have misspelled the method by adding a hard-to-spot third
trailing underscore.
2) __getattr__ is called when normal lookup fails, so the condition
evaluates always to False.
[color=blue]
> def __getattribute_ _(self, ppt):
> """getattribute """
> if hasattr(self, ppt):
> return self.ppt
> else:
> return "my custom computed result"[/color]

1) __getattribute_ _ is called for new-style classes only (those that
inherit directly or indirectly from object).
2) Even if your class was new-style, this would enter an infinite loop
because 'self.ppt' calls __getattribute_ _ again.

George

[Richard Copeland]

Re: How to customize getattr(obj, prop) function ?

I tested the exact code I sent you on python 2.4.1. What version of
Python are you using?

Thanks,
Rick

Pierre wrote:[color=blue]
> Nop I get the same AttributeError when I try to access to the property
> "c"...
>[/color]

[Richard Copeland]

Re: How to customize getattr(obj, prop) function ?

I tested the exact code I sent you on python 2.4.1. What version of
Python are you using?

Thanks,
Rick

Pierre wrote:[color=blue]
> Nop I get the same AttributeError when I try to access to the property
> "c"...
>[/color]

[Richard Copeland]

Re: How to customize getattr(obj, prop) function ?

I tested the exact code I sent you on python 2.4.1. What version of
Python are you using?

Thanks,
Rick

Pierre wrote:[color=blue]
> Nop I get the same AttributeError when I try to access to the property
> "c"...
>[/color]

[Pierre]

Re: How to customize getattr(obj, prop) function ?

I'm using python 2.4.3.

I removed the method __getattribute_ _

The mistake was my 3rd underscore :S sorry for wasting your time...

The problem is solved ! Thx all !

PS: Richard youre message are displayed 3 times ?!

[bruno at modulix]

Re: How to customize getattr(obj, prop) function ?

Pierre wrote:[color=blue]
> Hi,
>
> Sorry in advance, english is not my main language :/
>
> I'd like to customize the result obtained by getattr on an object : if
> the object has the requested property then return it BUT if the object
> doesn't has actually this property return something else.[/color]

So implement __getattr__(sel f, name).
[color=blue]
> In my case, I can't use getattr(object, property, default_value).[/color]

Why so ?
[color=blue]
>
> I tried to write a class with a __getattr__ method and even a
> __getattribute_ _ method but this doesn't do what I want....[/color]

Warning: __getattribute_ _ will take over normal attribute lookup. It can
be really tricky. Anyway, you don't need it for your use case - just
stick with __getattr__() and you'll be ok.
[color=blue]
> Maybe I didn't correctly understand this :
> http://docs.python.org/ref/attribute-access.html
>
> Here is a piece of my code :
> =============== =============== =======
> class myclass:[/color]

Make this :
class MyClass(object) :

[color=blue]
> """docstrin g"""
>
> a = 'aa'
> b = 'bb'[/color]

a and b are class attributes, not instance attributes. If you want
per-instance attributes (which is the most usual case...), you need to
define them in a method - usually the __init__() method:

def __init__(self):
self.a = 'aa'
self.b = 'bb'
[color=blue]
> def __getattr___(se lf, ppt):
> """getattr" ""
> if hasattr(self, ppt):
> return self.ppt[/color]

__getattr__() will be called only if the normal attribute lookup fails.
So calling hasattr(self, ...) in __getattr__ is useless. More than
this: the fact that __getattr__() has benn called means that normal
lookup has already failed, so hasattr() will end up calling
__getattr__().. .
[color=blue]
> else:
> return "my custom computed result"[/color]





(snip)
[color=blue]
>
> if __name__ == "__main__":
>
> d = myclass()
> p1 = getattr(d, "a")[/color]

You do understand that getattr(d, "a") is the same as d.a, don't you ?
[color=blue]
> print p1
> p2 = getattr(d, "b")
> print p2
> p3 = getattr(d, "c")
> print p3
> =============== =============== ==
>
> I get an AttributeError when accessing to the property named "c".
>
> Any explanation/solution to my problem ?[/color]

1/ use new-style classes
2/ only implement __getattr__()


here's a minimal working example:

class MyClass(object) :
def __init__(self, default='defaul t'):
self.a = 'aa'
self.b = 'bb'
self._default = default

def __getattr__(sel f, name):
return self._default

m = MyClass()
m.a
m.b
m.toto

HTH
--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom. gro'.split('@')])"

[Pierre]

Re: How to customize getattr(obj, prop) function ?

I don't want to use getattr(object, property, default_value) because
I'm using external code and I don't want to modify or patch it. In this
code, the call is getattr(object, property).

On my objects, I must provide default values depending on the property
that was requested, the default value is not always the same.

And Yes I understand that obj.a is equivalent to getattr(obj, 'a') BUT
the difference between class attribute and instance attribute... :S

[bruno at modulix]

Re: How to customize getattr(obj, prop) function ?

Pierre wrote:[color=blue]
> I don't want to use getattr(object, property, default_value) because
> I'm using external code and I don't want to modify or patch it. In this
> code, the call is getattr(object, property).[/color]

Seems like a perfectly valid reason !-)
[color=blue]
> On my objects, I must provide default values depending on the property
> that was requested, the default value is not always the same.[/color]

On what does it depends ? Attribute name ? Class ? Phase of the moon ?
[color=blue]
> And Yes I understand that obj.a is equivalent to getattr(obj, 'a') BUT
> the difference between class attribute and instance attribute... :S[/color]

If you mean you don't understand the difference between a class
attribute and an instance attribute, then it would be time to learn
Python's OO 101 - else you're in for trouble.

For short, an instance attribute has a per-instance value and is
(usually) stored in the object's __dict__, while a class attribute is
shared by all instances of the class and is (usually) stored in the
class's __dict__.


class Parrot(object):
cls_attr = 'class attribute'

def __init__(self):
self.instance_a ttr = 'instance attribute'

import pprint
pprint.pprint(P arrot.__dict__. items())
p = Parrot()
pprint.pprint(p .__dict__.items ())

print Parrot.cls_attr
try:
print Parrot.instance _attr
except AttributeError, e:
print e
# will lookup 'cls_attr' in p, then in Parrot
print p.cls_attr
print p.instance_attr

# will create an instance attribute 'cls_attr' in
# object p, shadowing Parrot.cls_attr
p.cls_attr = 'WTF ?'
print p.cls_attr
print Parrot.cls_attr

del p.cls_attr
print p.cls_attr
print Parrot.cls_attr

HTH
--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom. gro'.split('@')])"

[Ben Finney]

Re: How to customize getattr(obj, prop) function ?

[Please provide some context in your reply, so we know what questions
or comments you're replying to.]

"Pierre" <meyer.p@gmail. com> writes:
[color=blue]
> I don't want to use getattr(object, property, default_value) because
> I'm using external code and I don't want to modify or patch it.[/color]

You can subclass the class you want to modify, and override its behaviour.

class ScaryExternalTh ing(object):
""" The external class we don't want to modify. """

class OurModifiedFrie ndlyThing(Scary ExternalThing):
""" Class for use by our code. """

attr_defaults = {
'foo': "eggs",
'bar': "beans",
}

def __getattr__(sel f, name):
""" Method invoked when getting an unknown attribute """

value = attr_defaults.g et(name, "spam")
return value

--
\ "Welchen Teil von 'Gestalt' verstehen Sie nicht? [What part of |
`\ 'gestalt' don't you understand?]" -- Karsten M. Self |
_o__) |
Ben Finney